An electrode material with which thermal resistance, corrosion resistance, and thermal conductivity can be increased without using a noble metal or a noble metal alloy has been proposed as an electrode material for a center electrode and a ground electrode of an ignition plug (see, for example, Japanese Unexamined Patent Application Publication No. 5-114457).
In recent years, to increase the fuel efficiency of a vehicle and meet emissions regulations that have become more and more severe every year, an air-fuel ratio in the lean range, in which the air-fuel ratio is lower than the stoichiometric air-fuel ratio, has been commonly used as the air-fuel ratio while the vehicle is moving. To increase the fuel efficiency of a vehicle and meet emissions regulations, the air-fuel mixture is desirably completely combusted irrespective of the air-fuel ratio. Therefore, it is desirable to increase the ignitability of an air-fuel mixture having an air-fuel ratio lower than the stoichiometric air-fuel ratio. To achieve this, for example, a current (energy) applied to the ignition plug has been increased to increase the size of the spark generated at the time of ignition, a time period for which electricity is supplied to the ignition plug has been increased, and the fuel has been directly injected into a combustion chamber.
The increase in the size of the spark and the time period for which electricity is supplied tend to cause sway of the spark. When the direct injection technology is used, fuel injection may be performed a plurality of times within a single cycle, and the air-fuel mixture may flow at a high speed or in a complex manner in the combustion chamber depending on the ignition timing. In this case, the frequency of a ground electrode being affected by sway of the spark increases, and the degree of erosion of the base material of the ground electrode increases accordingly. As a result, there is a risk of misfiring due to separation of a noble metal chip bonded to the ground electrode or breakage of the ground electrode. In particular, erosion of a base portion of the ground electrode leads to a breakage of the ground electrode, resulting in a reduction in the performance of the ignition plug. When the ground electrode is protected simply by being coated with a noble metal or the like, the cost thereof is increased. The related art does not sufficiently address these problems.
There is still room for improvement in terms of the structure of the ground electrode with which uneven wear of the base material of the ground electrode can be effectively prevented or reduced. In particular, it is desirable to reduce uneven wear of the base material of the ground electrode without using a noble metal or a noble metal alloy. Furthermore, in the ground electrode structure including a noble metal chip, the structure for preventing or reducing uneven wear of the base material of the ground electrode and satisfactory bondability between the ground electrode and the noble metal chip have not been sufficiently studied.
Accordingly, there is a demand for an ignition plug in which erosion and uneven wear of a ground electrode can be prevented or reduced without using a noble metal or a noble metal alloy. There is also a demand for an ignition plug in which the occurrence of separation between the ground electrode and a noble metal chip can be prevented or reduced.
The present invention has been made to solve at least one of the above-described problems. Aspects of the present invention will now be described.